A MAX function can be described using the following expression:
                    MAX        ⁡                  (                      a            ,            b                    )                            =                      a        ,                                                  if            ⁢                                                  ⁢            a                    >          b                ,        or                                                                                        b        ,                                                  if            ⁢                                                  ⁢            b                    ≥          a                ,            where a and b are inputs. In particular, the output of the MAX function is equal to a if the value of a is greater than b, and the output is equal to b if the value of b is equal or greater than a.
Circuits implementing the MAX function are commonly used in applications to sense inputs corresponding to one or more physical properties and generate an output signal which corresponds to the highest value of the inputs. Examples of physical properties include, but not limited to, temperature, humidity, length and pressure. The physical properties may be sensed using suitable sensors which translate values of the physical properties to electrical signals such as current or voltage.
An example of an application of a circuit implementing the MAX function is monitoring heat produced by electrical circuits in electrical appliances. As operating frequencies and transistor densities continue to increase, the heat produced is able to permanently damage the electrical components in the electric circuits. Therefore, many electrical appliances have temperature sensors in them to monitor the amount of heat produced, and to reduce the heat if the temperature exceeds a certain value. When the temperature detected by the temperature sensors exceeds a certain value, the circuit may increase fan speed though additional circuitries or decrease the operating frequencies of any microprocessors to reduce the amount of heat produced.
In some electrical appliances such as networking products, the electrical circuits may be large, and hence, have different thermal characteristics at different points. Therefore, two temperature sensors can be used to sense temperature at two different parts of the appliances. The need to increase fan speed or other measures to reduce ambient temperature is determined based on the higher of the two temperature sensed by the temperature sensors.
A conventional method of generating the MAX function is to use sensors to generate input voltages corresponding to the values of the physical property such as temperature. A bank of rectifier diodes is subsequently used to generate an analog output voltage that is equal to the highest input voltage. This method requires that both inputs be voltage sources with a magnitude significantly higher than the forward voltage drop of the rectifier diodes. Another method is to use an analog multiplexer and an A/D converter followed by a microcontroller. This method is complex, and again, requires the inputs to be voltage sources.